1. Field of the Invention
This invention relates to so-called molecular gas laser oscillators in which laser transitions occur among lowlying vibrational levels in the ground electric state such as CO laser oscillator, a N.sub.2 O laser oscillator and a CO.sub.2 laser oscillator. More particularly, it relates to a gas laser oscillator which is improved to protect the oscillation characteristic, etc. from being adversely effected by the absorption of the laser beam.
2. Description of the Prior Art
A variety of gas laser oscillators such as a CO laser oscillator, a N.sub.2 0 laser oscillator, and a CO.sub.2 laser oscillator have been proposed and are known in the art.
The CO.sub.2 laser oscillator will be described by way of example. The amplification and output characteristics of various CO.sub.2 lasers have been investigated in detail; however, the effect of the above-described light absorption action on the output characteristic is not generally well known. The reason is that the effect of the light absorption action on the output characteristic of a low pressure (lower than about 50 torr) continuous oscillation CO.sub.2 laser or a pulsed CO.sub.2 laser is less. However, the light absorption action greatly affects the output characteristic of a high pressure continuous oscillation CO.sub.2 laser as described herein.
First, the relation between the action of light absorption and the oscillation characteristic will be described with reference to a typical or simplified internal mirror CO.sub.2 laser oscillator. The arrangement of the oscillator is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a laser medium formed by discharge; 2a and 2b, non-discharge portions; 3, a total reflection mirror; 4, a partial reflection mirror; and, 5 a laser beam.
In general, when a gas mixture of CO.sub.2 --N.sub.2 --H.sub.e or the like is excited by discharge, then the population inversion is formed between the CO.sub.2 (0 0.degree. 1) and (1 0.degree. 0), and the laser medium 1 becomes active. When the total reflection mirror 3 and the partial reflection mirror 4 having a predetermined reflectance are disposed on both sides of the laser medium 1 in such a manner that they confront each other, then laser oscillation is caused to emit the laser beam 5 from the oscillator.
In general, the laser output P is: ##STR1##
Where; A is the sectional area of the laser beam, Is is the saturation parameter, t is the transmission of the partial reflection mirror 4, g.sub.o is the unsaturated gain of the laser medium 1, l is the length, in the optical axis direction, of the laser medium, and a is the total loss of the resonator.
Consider now the total loss a of the resonator in the equation (I). The total loss a includes the entire diffraction loss of the resonator, the laser beam absorption and scattering loss of the two confronted mirrors, and the CO.sub.2 molecule absorption loss of the non-discharge portions. Among the three kinds of loss, the former two kinds of loss depend on the geometrical arrangement of the resonator and both the surface accuracy and the materials of the mirrors. However, the remaining loss, or the CO.sub.2 absorption loss, depends on the mixing ratio of the gas mixture, the gas temperature, and the total pressure. However, in the case where the total pressure is higher than 50 torr, i.e., in the pressure range where line width of the absorption spectrum is dominated by the collision broadening the absorption loss is independent of the total pressure.
Shown in FIG. 2 are the CO.sub.2 molecule absorption coefficients on the P(20) line in the CO.sub.2 10.4 .mu.m band which are calculated as a function of gas temperature. The absorption loss is represented by the product of the absorption coefficient and the length of the non-discharge portion. The absorption coefficient increases abruptly with increasing gas temperature at near room temperature, and has a tendency to saturate at near 600.degree. K. In the case when the gas temperature is maintained unchanged, the absorption coefficient increases in proportion to the mole fraction of CO.sub.2 molecules.
If, when the length of the non-discharge portion is for instance, 30 cm, the gas temperature is increased, then the absorption loss becomes about 8% from FIG. 2. In the case of an ordinary low pressure continuous oscillation CO.sub.2 laser, even if the non-discharge portion is heated by the CO.sub.2 molecule absorption attributed to the laser oscillation, the effect of the absorption loss on the laser output is small because g.sub.o l of the equation (1) has a value more than several hundreds of percent (%) and t can be selected large (t&gt;30%). In practice, the total pressure of the mixture gas used is of the order of 20 torr. Therefore the heat diffusion coefficient of the gas is increased, and the gas temperature increase due to the laser oscillation is limited. Thus, it can be considered that the absorption loss is much smaller than 8%. In the case of the pulse oscillation CO.sub.2 laser, the increase of the gas temperature is very small. For instance, if the gas temperature is 300.degree. K., then the absorption loss is about 0.5%. The loss of this order will not significantly affect the laser output.
As was described above, in the case of the ordinary low pressure continuous or the pulse oscillation CO.sub.2 laser, the effect of the absorption loss on the laser output is small.
Now, the high pressure continuous oscillation CO.sub.2 laser will be described. In view of the following, the laser oscillator of this type has been developed primarily as a laser oscillator small in size and high in output, higher than 1 KW. A laser output obtainable from a unit volume of the laser medium 1 is represented by g.sub.o.Is. When the gas pressure is higher than about 50 torr, then g.sub.o is inversely proportional to a gas pressure, and IS is proportional to the square of the gas pressure. Thus, g.sub.o.Is is proportional to the gas pressure.
According to this principle, the laser output is increased as the gas pressure increases. Thus, a high output can be obtained without increasing the size of the oscillator. This method of increasing the gas pressure is extremely effective in practical use. However, since g.sub.o is inversely proportional to the gas pressure, g.sub.o becomes smaller if the gas pressure is increased. Therefore, the value t to maximize the laser output also becomes smaller. In the case when the gas pressure is 300 torr and the laser output is 1 KW, g.sub.o.l is of the order of several tens of percent (%) and the value t to maximize the laser output is of the order of 10%. Conversely, it can be considered that as the gas pressure is high, the heat diffusion coefficient of the gas is reduced, and the gas temperature of the non-discharge portion is considerably increased upon laser oscillation. Thus, under the above-described conditions, the absorption loss is of the order of 8%. If these conditions are applied to the equation (1), then it can be estimated that the absorption loss greatly adversely affects the laser output.
With the above-described knowledge, the high pressure continuous oscillation CO.sub.2 laser oscillation will now be described in detail.
A conventional laser oscillator of this type is shown in FIG. 3. In FIG. 3, reference characters 6a, 6b and 6c designate blowers for creating gas flows. Arrows 7 indicate the gas flows. A heat exchanger 8 is employed for cooling the mixture gas which has been heated by discharge. Elements 9a and 9b are bellows for changing the angles of the mirrors. And element 10 is a container for sealing the gas mixture therein.
This laser oscillator is a so-called three orthogonal axis type, in which the direction of the laser beam axis, the gas flow and the discharge are mutually perpendicular. The gas mixture containing CO.sub.2 molecules in the container 10 is formed into the gas flow by the blowers 6a, 6b and 6c, the gas flow being through the discharge portion and the heat exchanger 8 to the blowers. As is well known in the art, the gas flow is necessary to prevent the reduction of the laser output due to the increase in temperature of the laser medium 1. The velocity of the gas is, in general, in the order of 30 m/sec. The laser medium 1 is formed by glow discharge in a direction perpendicular to the surface of the figure. The total reflection mirror 3 and the partial reflection mirror 4 are disposed on both sides of the laser medium 1, and the laser beam 5 is emitted from the partial reflection mirror 4.
The curve A in FIG. 4 indicates the variations of the laser output with respect to discharge inputs under the following conditions:
Gas mixture: CO.sub.2 --CO--N.sub.2 --He=2--1--6--32 PA0 Total pressure: 300 torr PA0 Gas velocity: 30 m/sec PA0 Discharge length: 80 cm PA0 Reference of partial reflection mirror's reflection factor: 80%
The laser output rises linearly at the oscillation threshold, and is then saturated in a certain discharge input region C (FIG. 4). Thereafter, the laser output is increased linearly. If it is assumed that the absorption loss of the non-discharge portion does not affect the laser output, then it can be considered from equation (1) that the laser output increases linearly with increasing discharge input. This is because g.sub.o is substantially proportional to the discharge input when the gas temperature of the laser medium is low (the actual gas temperature being lower than 400.degree. K.). The phenomenon where the laser output is saturated in the discharge input region is closely related to the absorption phenomenon of the CO.sub.2 molecules. However, the detailed description of this relation will be omitted since it is not directly related to the present invention.
In the conventional high pressure continuous oscillation CO.sub.2 laser oscillator, non-discharge portions 2a and 2b are provided as shown in FIG. 3, and the absorption loss in these portion adversely affects the laser output. As described herein, the output of a laser oscillator of this invention is higher by about 40% than that of the conventional one. As was described before, the local gas temperature of the non-discharge portions becomes very high, and therefore, the gas temperature in the portions fluctuates spatially and with time. As a result, the absorption loss is varied and the stability of the laser output is lost. The actual output variation percentage due to this fluctuation is about 10% at a laser power of 1 KW. Furthermore, the temperature of the gas mixture in contact with the surfaces of the total reflection mirror 3 and the partial reflection mirror is greatly increased. These mirrors are heated through heat conduction of the gas. Since the partial reflection mirror 4 is made of Ge, GaAs or ZnSe, if the temperature is increased, then the mirror is broken by the heat stress which is caused inside the mirror.
For the gas laser oscillator, especially the high pressure continuous oscillation gas laser oscillator, it is necessary to increase the oscillation efficiency and the stability of the laser output.